Whisk together the flour, sugar, and salt in a large bowl or the bowl of your food processor. (I used my food processor, but I didn't always have one and have had great results making this crust by hand.) Please note that this is basically a double pie, meaning it is a lot of dough! I did mine in two batches-that is, I divided the recipe in half and repeated the process twice. If you have a giant mixing bowl or food processor, go for it, but my equipment isn't large enough to handle 7 1/2 cups of flour.,Add in the chopped pieces of cold butter and cold shortening. Blend together with your hands or pulse with your processor until you have coarse crumbs (it doesn't have to be perfect). You can use a pastry cutter, too, but I find that a clean pair of hands work best. Mix together the water and vinegar in a small bowl. When ready, slowly drizzle it over the dough, a tablespoon or so at a time, gently stirring the mixture with a fork or pulsing with your processor, until fully incorporated (you may not use all of the liquid).,It might seem a bit too wet at this point, but it will dry up while it sits in the fridge. Form the dough gently into 2 loose balls, wrap tightly in plastic wrap, and chill in the fridge for a minimum of 3 hours or overnight (as always, overnight is best). Make your filling when you are almost ready to use your dough.,______,Meanwhile, place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment. Start whipping the egg whites to soft peaks on medium speed. You want to have the egg whites whipped and ready to go, waiting for your syrup to be drizzled in. If they're whipping faster than your syrup is coming to temperature, just stop the mixer until the syrup is ready, which is what I did.,When the syrup reaches 240° F, reduce the mixer speed to low and slowly drizzle about 2 tablespoons of syrup into the egg whites to warm them. If you add too much syrup at once, the whites will scramble. Slowly drizzle in the rest of the syrup, a bit at a time-seriously, do not do more than 2 tablespoons at a time.,Increase the speed to medium high and whip until the marshmallow fluff is stiff and glossy, about 7 solid minutes. Don't try to speed this process up! Add in the vanilla and whip 2 minutes more. Use the fluff immediately or refrigerate stored in an airtight container for up to 2 weeks. (I made my fluff about 4 hours ahead of time before baking with it, and I actually doubled the recipe so that I'd have some extra to spread of the top of the pie pieces... just an idea!),For the chocolate filling: If you have a double boiler, use that. I don't, so I just melt chocolate very slowly and carefully because it burns easily. In a saucepan, combine your cocoa powder, heavy cream, chocolate, sugar, butter, and salt. Stir constantly medium-low heat constantly until everything is melted and smooth. Remove from the heat and set aside. The mixture will thicken up as it cools down, making it easy to spread over your pie crust later.,For assembly: Remove your crust from the refrigerator to allow for easier rolling. Preheat your oven to 400° F. Generously butter a half-sheet pan. Roll out your first ball of dough-it should be about an inch or two longer and wider than the baking sheet, but it does not have to be perfect by any means. If you need to re-roll a few scraps here and there to patch up pieces, that's fine. Move it over onto the baking sheet by draping it over the rolling pin. Unfold it and gently press the pie crust into the bottom of the baking sheet.,Spread your chocolate mixture over the dough, then scatter mini marshmallows over top, saving some for a finishing touch once the pie is baked. Sprinkle 4 crushed graham crackers over the top of the marshmallow. Roll out your second pie crust and gently place it over the top. Crimp the edges together, discarding any scraps (or using them to decorate the pie, dealer's choice). Pull the dough slightly inward, away from the edges of the baking sheet, to make it easier to cut later.,Use a fork to make a pattern in the top of the pie, like a traditional Pop-Tart. Beat together your egg and water and brush the top of the pie with it. Place the pie in the oven, then reduce the heat to 375° F. Bake for 50 to 60 minutes, until golden brown and shiny on top. Remove from the oven and allow to cool completely.,Spread the marshmallow fluff all over the top of the pie and decorate with any leftover mini marshmallows. Use a kitchen torch to brulee your topping, then slice, serve, and enjoy!